There are several processs which causes intensity loss of X-rays passing through matter. For X-ray quanta of wavelengths longer than a few tenths of 1 .ANG., say 0.5 .ANG., a primary process of X-ray absorption is the photoelectric effect. In this process the absorbed radiation energy is used for ejection of a low level electron from an atom. The photoelectron carries away any excess energy in the form of kinetic energy.
Since an electron can only be ejected from an inner shell if the quantum of the incident (X-ray) photon exceeds its bound energy, the X-ray absorption varies discontinuously with wavelength (or energy). The probability of ejection is largest for quanta of energy just exceeding the bound energy (called the absorption edge of the electron level, e.g., K-edge for the K-level electrons); it is small for a photon of energy very much in excess of that of the absorption edge.
When the atoms are in gas phase, the absorption coefficients decrease smoothly beyond an absorption edge as the energy X-ray quanta increases, until it reaches another absorption edge. But if the atoms are in liquid or solid phases, the absorption coefficients oscillate as the X-ray energy increases for a range of 1000 ev or so beyond an absorption edge. Such oscillation structures in the absorption coefficients are called extended X-ray absorption fine structure (EXAFS). The structures near absorption edges are called X-ray absorption near-edge structures (XANES). Both EXAFS and XANES are due to the influence of the neighboring atoms to the photoelectrons. As such both EXAFS and XANES contain information about the atomic arrangement near the absorbing atom. Therefore EXAFS and XANES are tools for studying local structures of matter.
The conventional methods for structure determination are the diffraction methods using X-ray, neutron beams or electron beams. However they are applicable only to structures with regular atomic arrangement, such as crystals. For materials lacking regularity, the diffraction method is much less useful. One can determine the local configuration of only relatively simple molecules. Although some insight to more complicated molecules can be gained from optical spectroscopy and magnetic resonance techniques, these techniques provide only indirect evidence from which the structural parameters of interest must be inferred. These limitations can be overcome by using EXAFS. The major requirement of performing EXAFS measurements is the availability of intense X-ray with a continuous spectrum. Thus, EXAFS has become a popular tool for structure determination only since synchrotron radiation has become available since the early 1970's. XANES is measured exactly the same way as EXAFS. It is difficult to obtain quantitative structural parameters from XANES, but it is very sensitive to the local chemistry, such as electronic distribution. So far XANES have been used only for qualitative analysis of local structures.
There are two standard ways of measuring EXAFS (and/or XANES). One is the transmission method in which one measures the incident and transmitted intensities of X-ray. The other is the fluorescence method in which one measures the incident and fluorescence intensities of X-rays. Recently with the advance of synchrotron radiation (SR) producing techniques, the intensity of SR has become so strong that one can measure EXAFS in a very short period of time. This opens the possibility of measuring time-resolved EXAFS of dynamic systems, from which one can infer the time-sequence of structural changes in a dynamic system. This is particularly useful for dynamic systems with relaxation times in the range of tens of microseconds to milliseconds, such as proteins in low temperature and solid structural changes near critical temperatures.
But the conventional methods of measuring either transmission or fluorescence EXAFS are inefficient for time-resolved measurements. Typically, the transmission EXAFS measurements involve a voltage to frequency converter with a maximum frequency of 10.sup.6 Hz; therefore it is difficult to measure an EXAFS point in less than 10 millisecond. The typical way of measuring fluorescence EXAFS is to use a counter to measure fluorescence intensity. The problem here is that synchrotron radiation is produced in pulses, typically one pulse per microsecond. Each pulse can contain 10.sup.6 photons per ev. In many cases a counter would intercept more than one fluorescence photons produced by such a pulse. The counter would count only one pulse no matter how many more fluorescence photons are intercepted. Therefore, in order to measure the changes in the EXAFS amplitude, one has to arrange the counter in such a way that no more than one fluorescence photon per SR pulse would be intercepted. This means that synchrotron radiation is not used to its full potential and the time-resolution cannot be improved even if the intensity of synchrotron radiation is further increased. Accordingly, the present invention is an EXAFS spectrometer which integrates the transmission X-ray or fluorescence X-ray over a chosen period of time (e.g. 100 microseconds, 1 millisecond, etc.) and makes the full use of intense synchrotron radiation, thus improving the time-resolution of time-resolved EXAFS measurements and automatically increasing measurement precision with the intensity of synchrotron radiation.